Ultra-sensitive photoresponse and persistent photoconductivity in modulation doped Ge/SiGe and Si/SiGe heterostructures

Abstract

We have studied the effect of incident light on Ge/SiGe and Si/SiGe modulation doped heterostructures with both inverted and normal types of doping. We report for the first time an extremely sensitive photoresponse in modulation doped inverted Si/Ge x Si 1−x /Ge/Ge x Si 1−x two-dimensional hole gas (2DHG) heterostructures at low temperatures (∼10 K). The resistance of the 2DHG is found to decrease significantly upon illumination with 640 nm radiation of incident powers as low as ∼1 fW. The change in resistance persists after the illumination is switched off, indicating the persistent photoconductivity effect. The sensitivity of the sample to light exposure was measured over several orders of incident powers from ∼1 fW to 10 nW and found to increase with power. By comparing the sensitivity in different normal and inverted heterostructures it was concluded that the high sensitivity of the inverted structures is due to the high mobility of the photocarriers confined in the channel closer to the top interface.